As mobile terminals such as smartphones have become common, the number of base stations that connect mobile terminals to wireless communication networks have been increasing. Particularly, the number of small cell base stations that cover small cells overlapping macro cells is increasing, to compensate for local shortage of bandwidth of the macro cell base stations that cover the macro cells (see Japanese Laid-open Patent Publication No. 2011-103570 (hereinafter, referred to as Patent Document 1), for example).
In some cases, a passive optical network (PON) with low optical fiber installation costs is used as a mobile backhaul network that connects a large number of base stations to a device on a higher order (see International Publication Pamphlet No. WO 2014/103804 and Japanese Laid-open Patent Publication No. 2011-176555 (hereinafter, respectively referred to as Patent Documents 2 and 3), for example). The IEEE (the Institute of Electrical and Electronics Engineers, Inc.) 802.3ah specifies techniques relating to GE (Gigabit Ethernet)-PON of 1 Gps or higher, which is compliant with Ethernet (registered trademark, which also applies in the description below), for example.
A PON is a star-like access optical network in which an optical terminal device and optical terminating devices are connected via an optical coupler that splits light. An optical terminal device is called an optical line termination (OLT) or the like, and an optical terminating device is called an optical network unit (ONU) or the like. In a case where a PON is used as a mobile backhaul network, not only the costs for optical fiber installation can be lowered, but also can traffics of base stations be transmitted by a single OLT.